As the need to provide sophisticated circuitry in smaller packages grows, techniques such as "die stacking" are becoming increasing popular. With one method of die stacking, the die are separated by a spacer comprised of an insulating material, such as silicon aluminum nitride or alumina to provide clearance for the wire bond connections made between the bond pads at the top periphery of the die and and the mating bond pads of the supporting substrate.
Bypass capacitors are used in large and small circuits to provide local decoupling of the supply voltages around active circuitry. Placing bypass capacitors as close as possible to the active circuitry and minimizing the length of the power and ground connections between the bypass capacitors and the active circuitry optimizes power supply stabilization and noise suppression.
Optimal installation of bypass capacitors with sufficient capacitance is problematic in planar multichip assemblies to begin with, but is even more difficult with stacked die owing to the lengthening of the bond wires from the die to the substrate and higher active circuit densities. This is evidenced in FIGS. 4 and 5, which demonstrate an approach where bypass capacitors are attached on a mini-substrate that is attached to the top of the stack, a notable characteristic being that all the bypass currents flow through long bond wires 78, 80 (relative to the size of the circuit) because the capacitors 78 are at the top of the stack. Thus, when a bypass capacitor provides current to the bottom die 76 (when there is temporary drop in the supply voltage to the circuit), the stabilized current from the capacitor 70 flows through the connection 78 and then up through connection 80. The connection lengths are small compared to normal large circuits, but their impedances can have a significant effect under high current transients (di/dt) caused by the active circuitry.